I have several questions for you.
1. Why it it YOUR job to "protect" these men?
2. Have you, for some reason, decided that you must interfere with this other woman's sexuality ?
3. Do you have PROVEN information that this is a public health issue?
4. Or is it just that you find her behavior doesn't match with your morals?
I would suggest that you should stay out of this one. Let this woman and the men she sleeps with make their own decisions based on their own reasons!